1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, more particularly to a front lit LCD apparatus, and a portable electronic device incorporating such a liquid crystal display apparatus.
2. Description of the Related Art
Liquid crystal display apparatuses display characters and/or images by adjusting the amount of transmitted portions of light from a light source. The liquid crystal itself does not emit light, unlike CRT (cathode ray tube) devices, PDP (plasma display) devices, and EL (electroluminescence) devices.
Conventional liquid crystal display apparatuses are generally classified into transmission type liquid crystal display apparatuses and reflection type liquid crystal display apparatuses. A transmission type liquid crystal display apparatus incorporates a light source or “back light” of a planar configuration (e.g., a fluorescent tube or an EL device) provided behind an LCD (liquid crystal display) device.
A reflection type liquid crystal display apparatus has an advantage in that it performs a display function by utilizing ambient light. As a result, a reflection type liquid crystal display apparatus does not require a back light, and consumes relatively small power. Furthermore, reflection type liquid crystal display apparatuses provide improved visibility in very bright conditions (e.g., indirect sunlight) where a transmission type liquid crystal display apparatus or any emission type display device would provide poor visibility. Therefore, portable information terminal devices and mobile computers, which have enjoyed increasing demand in recent years, often employ reflection type liquid crystal displays.
However, a reflection type liquid crystal display apparatus has the following problems: since a reflection type display apparatus utilizes ambient light for display, its display luminance depends on the environment in which it is used. Accordingly, an image that is displayed on a reflection type display apparatus may become totally illegible in the dark, e.g., at night. This becomes particularly problematic for reflection type display apparatuses incorporating a color filter for color display functions because they require even more ambient light than monochrome display apparatuses.
As one solution to the above problem, a semi-transmission type liquid crystal display apparatus has been proposed which incorporates a half mirror as a reflector. However, half mirrors require a complicated manufacturing process. Moreover, since the structure of a half mirror in itself does not make for highly efficient utilization of light, a semi-transmission type liquid crystal display apparatus incorporating a half mirror does not provide excellent display quality.
Accordingly, a reflection type display apparatus has been proposed which employs a means for illuminating a reflection type display device from the front as an auxiliary illumination device in a dark environment.
An example of an auxiliary frontal illumination device is disclosed in Japanese Utility Model Registration No. 3003427. The disclosed illumination device includes an optical guide member which is disposed in front of an object to be illuminated so as to cover the entire face thereof, the optical guide member having a light controlling surface, as well as a cold cathode fluorescent tube disposed at an edge portion of the optical guide member.
Examples of back-lighting techniques are disclosed in Japanese Laid-Open Publication Nos. 6-123884 and 8-68997, for example. The 6-123884 publication discloses an optical guide member including stripe lenses extending in a direction at an angle with respect to the direction in which the pixels of a transmission type display device are arrayed, thereby minimizing the generation of moire fringes.
According to this technique, an image is perceived by a viewer as the illumination light from the back light source travels through the stripe lenses of the optical guide member and the pixels of the transmission type LCD so as to reach the eyes of the viewer. The pitch of the stripe lenses of the optical guide member and the period of the pixels of the transmission type LCD would normally interfere with each other to cause moire fringes. However, by retaining an angle between the stripe lenses of the optical guide member and the pixels of the transmission type LCD, this technique sufficiently shortens the period of the moire fringes to prevent the viewer from perceiving them.
Japanese Laid-Open Publication No. 8-68997 discloses a back light including two prism sheets such that an angle is retained between the direction of the prism grooves provided in two prism sheets and the direction along which pixels of a transmission type display device are arrayed, thereby minimizing the generation of moire fringes.
The illumination light from the back light travels through the two prism sheets and the pixels of the transmission type LCD before it reaches the eyes of a viewer, so that the interference between the periods of these three elements would normally generate moire fringes. According to this technique of retaining an angle between the three elements, the period of the moire fringes is sufficiently shortened to prevent the viewer from perceiving them. According to this structure, the angle between the two prism sheets and the pixel pattern of the transmission type LCD can be independently set within the range of 4° to 86°.
However, various problems may occur when any one of the above-described conventional illumination devices is employed as an illumination device (front light) for a reflection type LCD, as described below.
First, the illumination device disclosed in Japanese Utility Model Registration No. 3003427 is designed to illuminate objects such as drawings, pictures, or printed materials, which are usually unharmed by any moire patterns (moire fringes) emerging due to light interference. If this illumination device is employed as a front light of a reflection type LCD, light interference may occur between the optical guide member and the regular array of pixels of the reflection type LCD, thereby causing moire fringes. In the case where the optical guide member includes some periodic structure provided thereon, the colored portions (e.g., red (R), blue (B), and green (G)) of a color filter may intensify a “prism phenomenon” (under which light beams of red, green, and blue are separately perceived). As a result, the display quality may greatly deteriorate.
Furthermore, while disclosing a technique of leading light onto the object to be illuminated, the 3003427 publication fails to disclose a light controlling portion having any specific shape or structure. The shape and structure of a light controlling portion and location of a light source are important factors for realizing efficient guiding of light.
Now, the mechanism of moire fringe (stripe pattern) generation due to light interference will be briefly explained with respect to the case of employing an illumination device as a front light of a reflection type LCD.
As shown in FIG. 16, illumination light 2213 from a front light (light source) 2200 is reflected from a periodic structure (consisting of concave and convex portions) 2212 formed on an optical guide member 2211; reflected from the pixels of a reflection type LCD 2210; and allowed to pass back through the periodic structure 2212 of the optical guide member 2211. When a viewer observes the reflection type LCD 2210 against ambient light 2214, the ambient light 2214 will pass through the periodic structure 2212 of the optical guide member 2211 and will be reflected from the pixels of the reflection type LCD 2210 so as to pass back through the periodic structure 2212 of the optical guide member 2211.
The above-described structure may be considered as including three periodic patterns, namely, two periodic patterns which are defined by the two passages of light back and forth through the periodic structure 2212 of the optical guide member 2211, and one periodic pattern defined by the pixel pattern of the reflection type LCD 2210. Due to overlapping of these three periodic patterns, moire fringes may be generated.
Thus, a front light configuration, in which an optical guide member is disposed in front of a reflection type LCD, may allow moire fringes to be generated due to interference between three different periods. On the other hand, a back light configuration, in which an optical guide member is disposed behind a transmission type LCD, may allow moire fringes to be generated due to interference between two elements, i.e., the periodic structure on the optical guide member and the pixel pattern of the transmission type LCD.
Thus, the mechanism of moire fringe generation differs depending on whether the illumination device is employed as a front light for a reflection type LCD or as a back light for a transmission type LCD. Therefore, using either one of the back lights disclosed in Japanese Laid-Open Publication Nos. 6-123884 and 8-68997 as a front light for a reflection type LCD cannot effectively prevent the generation of moire fringes. Little flexibility will be provided with respect to the angular relationship between the aforementioned periodic structures.